Process for producing tin sintered bodies and coatings

ABSTRACT

TiN sintered bodies and coatings are produced by dispersing nanocrystalline TiN powder in water and/or a polar organic solvent as dispersing agent in the presence of at least one low molecular organic compound having at least one functional group which can react or interact with groups on the surface of the powder particles, removing the dispersing agent and sintering the surface-modified TiN which has been processed into green bodies or coatings before or after the removal of the dispersing agent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing sintered bodiesor coatings of titanium nitride (TiN) using a suspension ofsurface-modified TiN particles in the nanoscale range.

2. Discussion of the Background

In the following "particles in the nanoscale range" are to denoteparticles whose average size is not higher than 100 nm, particularly nothigher than 50 nm, and particularly preferred not higher than 30 nm.

In the processing of nanodisperse materials (particles, powders), thereare essentially two problems, i.e.:

(a) control of particle agglomeration in the processing of saidmaterials; and

(b) production of processable ceramic compositions having a high solidscontent.

Regarding problem (a) it is to be noted that when going from ceramicpowders in the sub-micron range to those in the nanoscale range, anincrease in agglomeration is generally observed. This is due to the factthat with decreasing particle size, weak interactive forces such as vander Waals forces gain significant importance and dominate, respectively.Additionally, there are always functional groups, i.e., groups capableof undergoing condensation, on the particle surface. With conventionalsub-micron powders, said forces are only of importance as far as theymay be used as centers of interaction for necessary organic processingaids (dispersing aids, binders, etc.). Due to the high surface to volumeratio in nanodisperse materials, said surface groups are, however, alsoof great importance in another respect. On the one hand, they can alsoserve as reaction centers for organic processing aids. However, bycondensation reactions between individual particles they can also resultin the formation of hard agglomerates. Said particles are then connectedto each other quasi by "sinter necks". Therefore, it is desirable todevelop processes with which the agglomeration can be controlled suchthat agglomerated powders can be obtained in a controlled manner.Further, it would be favorable if by said process the reactive surfacecould be shielded outwardly, thus preventing a condensation betweenparticles.

As regards the above problem (b) it may be noted that the production ofceramic compositions having high solids contents and processingproperties adapted to a molding process causes severe difficulties. Inorder to avoid agglomerates which may result in serious defects both inthe green and sintered bodies, the operation is usually carried out insuspension. For stabilizing the suspension, dispersion aids are addedwhich dispersion aids have the function of preventing agglomeration andimparting the required processing properties to the suspension. In thestabilization of the suspension there are two different basicpossibilities, i.e., electrostatic and steric stabilization.

The electrostatic stabilization is disadvantageous in that due to thehigh hydrodynamic radius of the suspended particles only low solidscontents may be realized. In contrast thereto, the steric stabilizationon principle offers the possibility of preparing suspensions of highsolids contents from materials in the nanoscale range since in this casethe hydrodynamic particle radius is much smaller.

The advantages of steric stabilization have already been demonstratedwith nanodisperse SiO₂ as example. In this case, non-ionic organicpolymers (e.g., polymethyl methacrylate) which become adsorbed onto theparticle surface have generally been employed. The disadvantage of sucha stabilization is that even therewith only solids contents of at themost about 20 to 30% by volume may be realized and that saidstabilization can be applied to systems different from SiO₂ only withconsiderable restrictions. The reason therefor mainly is that thechemical properties relating to the surface (e.g., acidic/basicproperties) specific for a particular material cannot be taken intoaccount.

Thus, it is desirable to provide a process which makes it possible tomodify the particle surface by corresponding chemical compounds so thatan optimum degree of dispersion and high solids contents of thedispersion may be realized.

Titanium nitride (TiN) belongs to the group of metallic hard materialsand has a cubic crystal structure. Due to its high proportion ofcovalent bonding, TiN has a high melting point, a high hardness as wellas a high resistance to oxidation and corrosion. Said properties resultin the application of TiN as layer material for protecting metalsagainst wear, and as one of the components of multiple-phase ceramicssuch as Al₂ O₃ /TiN or Si₃ N₄ /TiN.

Currently, coatings of pure TiN and TiN coatings with added TiC,respectively are prepared via gas phase processes. Said processesinclude the CVD (chemical vapor deposition) and the PVD (physical vapordeposition) processes. Corresponding equipment is commercially availableand is a part of industrial production processes. Said coatings areemployed in the following fields:

protection of metals against wear in abrasive and tribologicalapplications,

on cutting, drilling and milling tools in order to increase themachining performance,

as corrosion protection coatings in chemical reactors,

as coatings of clock casings and jewelry.

One disadvantage of TiN coatings produced via CVD and PVD is theinsufficient adhesion to the substrates so that said coatings frequentlychip off and tools coated therewith become unservicable prematurely.Employable substrates are metals having a high heat resistance, hardmetals such as, e.g., WC/Co, or also ceramic indexable inserts.

A further application of TiN relates to the use in mixed ceramics suchas, e.g., Al₂ O₃ /TiN or Si₃ N₄ /TiN. By adding TiN to said matrixmaterials the mechnical properties thereof, such as hardness, tenacityor compression strength, may be improved. The proportion of TiN in saidcomposites may amount to up to 20% by volume. An application of TiN asbulk material is not currently known.

Due to its high covalent bonding character, pure TiN only has a very lowsintering activity. Therefore, densification thereof usually requiresthe use of sintering additives. In the simplest case said sinteringadditive may be TiO₂ which is formed in air on the surface of TiN in thepresence of water. Thus it has, for example, been reported that thepressureless sintering of TiN of medium grain size of 0.1 μm attemperatures of 1500° C. results in relative densities of up to 95%.Said sintering performance is attributed to the activation of thediffusion mechanisms via the dissolution of TiO₂ localized on thesurfaces of the TiN particles, which mechanisms result in densification.

Various publications deal with the sintering of TiN under pressureand/or in the presence of sintering additives. Thus, the hot-pressing ofTiN powders having a d₅₀ value of 1 μm at temperatures up to 2100° C.and a pressing pressure of 14 MPa leads to a density of only 93% of thetheory; cf. M. Morijama et al., "Mechanical and Electrical Properties ofHot-pressed TiN-Ceramics without Additives", J. Jap. Ceram. Soc., 99(1991). In M. Morijam et al., "The Mechanical Properties of Hot-pressedTiN-Ceramics with Various Additives", J. Jap. Ceram. Soc., 101 (1993),the sintering performance of TiN in the presence of sintering additivesin a hot-pressing operation is described. After hot-pressing at 1950° C.and 14 MPa, samples containing a total of 10% by wt. of Al₂ O₃, Y₂ O₃and B₄ C result in a density of around 97% of the theory. Furthermore, a95% densification by hot-pressing at 1800° C. and 5,0 GPa has beenreported.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a process forproducing TiN sintered bodies and coatings which makes it possible tocontrol particle agglomeration and to obtain sufficiently high solidscontents of the particle suspensions employed and which can be carriedout at lower sintering temperatures.

According to the present invention said object is achieved by a processfor producing TiN sintered bodies or coatings which is characterized inthat nanocrystalline TiN powder is dispersed in water and/or a polarorganic solvent as dispersion medium in the presence of at least one lowmolecular weight organic compound having at least one functional groupcapable of reacting and/or interacting with groups present on thesurfaces of the powder particles, said dispersion medium is removed andsaid surface-modified TiN which prior to or after the removal of thedispersion medium has been processed into green bodies or coatings issintered.

The process according to the present invention makes it possible tocontrol the agglomeration of TiN powders in the nanoscale range, thusmaking it possible to produce dispersions of such particles at highsolids contents in a satisfactory manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

TiN powder having a primary particle size of preferably below 100 nm isparticularly suitable as starting material for the process according tothe present invention. Said powder in the as-supplied-form is stronglyagglomerated and has agglomerates of a size of several 10 μm. Further,said powder is known to be present in the crystalline form of osborniteand to have a thin layer of TiO₂ on the particle surface.

In order to deagglomerate the agglomerates of the TiN starting materialin the dispersion medium into their primary particles and to prepare astable nanodisperse suspension, according to the present inventionsurface-modifying agents, i.e., surface-modifying low molecular weightorganic (=carbon containing) compounds having at least (and preferably)one functional group capable of reacting and/or (at least) interactingwith groups present on the surfaces of the TiN particles are employed.Particularly, compounds having a molecular weight not higher than 1000,preferably not higher than 500, and particularly not higher than 350 aresuitable for said purpose. Preferably, such compounds are liquid undernormal conditions and soluble or at least emulsifyable in the dispersionmedium.

Such compounds preferably have not more than a total of 30, particularlynot more than a total of 20, and particularly preferred not more than 15carbon atoms. The functional groups which said compounds are required tohave are in the first place determined by the surface groups of therespective TiN starting material employed and, additionally, by thedesired interaction. It is particularly preferred if anacid/base-reaction according to Bronsted or Lewis (including complexformation and adduct formation) can take place between the functionalgroups of the surface-modifying compound and the surface groups of theTiN particles. An example of a further suitable interaction is thedipole-dipole-interaction. Examples of preferred functional groups thusare carboxylic acid groups, (primary, secondary and tertiary) aminogroups and C-H-acidic groups. Also, several of said groups may bepresent simultaneously in a molecule (betaines, amino acids, EDTA,etc.).

Accordingly, examples of particularly preferred surface-modifying agentsare saturated or unsaturated mono- and polycarboxylic acids (preferablymonocarboxylic acids) having 1 to 12 carbon atoms (e.g., formic acid,acetic acid, propionic acid, butanoic acid, pentanoic acid, hexanoicacid, acrylic acid, methacrylic acid, crotonic acid, citric acid, adipicacid, succinic acid, glutaric acid, oxalic acid, maleic acid and fumaricacid). If unsaturated carboxylic acids are employed there is theadditional possibility of carrying out a crosslinking reaction by meansof the ethylenically unsaturated double bond.

Examples of further suitable surface-modifying agents are mono- andpolyamines, particularly those of the general formula R_(3-n) NH_(n),wherein n=0, 1 or 2, and the radicals R independently are alkyl groupshaving from 1 to 12, particularly from 1 to 6, and particularlypreferred from 1 to 4 carbon atoms (e.g., methyl, ethyl, n- and i-propyland butyl) and ethylene polyamines (e.g., ethylene diamine, diethylenetriamine, etc.); β-dicarbonyl compounds having from 4 to 12,particularly from 5 to 8 carbon atoms, such as, e.g., acetylacetone,2,4-hexanedione, 3,5-heptanedione, acetoacetic acid, and acetoaceticacid C₁ -C₄ alkyl esters; organoalkoxy silanes, such as those employedfor the surface-modification of colloidal silica (e.g., those of thegeneral formula R_(4-m) Si(OR')_(m), wherein the groups R and R'independently represent C₁ -C₄ alkyl and m is 1, 2, 3 or 4); andmodified alcoholates, wherein the OR groups (R as defined above) arepartially substituted by inert organic groups, and bonding(condensation) onto the particle surface takes place through the stillpresent OR groups and said organic groups take care of the shielding.Examples thereof are zirconium and titanium alcoholates M(OR)₄ (M=Ti,Zr), wherein the groups OR are partially replaced by complex formingagents such as a β-dicarbonyl compound or a (mono)carboxylic acid. If anethylenically unsaturated compound (such as, e.g., methacrylic acid) isused as complex forming agent, additionally also a crosslinking reactioncan take place (see above).

Particularly preferred surface-modifying agents are guanidine carbonateand guanidinopropionic acid.

As dispersion medium, water and/or polar organic solvents are employed.As polar organic solvents, preferably those which are miscible withwater are suitable. Specific examples of employable polar organicsolvents are alcohols such as, e.g., aliphatic alcohols having 1 to 6carbon atoms (particularly methanol, ethanol, n- and i-propanol andbutanol); ketones such as, e.g., acetone and butanone; esters such as,e.g., ethylacetate; ethers such as, e.g., diethylether, tetrahydrofuranand tetrahydropyran; amides such as, e.g., dimethylacetamide anddimethylformamide; sulfoxides and sulfones such as, e.g., sulfolane anddimethylsulfoxide; and halogenated aliphatic hydrocarbons. Mixtures ofsaid solvents may, of course, also be employed.

The dispersion medium employed preferably has a boiling point whichmakes it possible to remove said dispersion medium by distillation(optionally under reduced pressure) without any problems. Solventshaving a boiling point below 200° C., particularly below 150° C., arepreferred.

In practicing the process according to the present invention, theconcentration of dispersion medium generally is 20 to 90% by wt.,preferably 30 to 80% by wt., and particularly 35 to 75% by wt. Thebalance of the dispersion is constituted of TiN starting powder and lowmolecular weight organic compound (surface-modifying agent). In thiscase the weight ratio TiN powder/surface-modifying agent generallyranges from 1000:1 to 4:1, particularly from 500:1 to 8:1, andparticularly preferred from 250:1 to 10:1.

The process according to the present invention is preferably carried outat a temperature of from room temperature (about 20° C.) to the boilingtemperature of the dispersion medium. Dispersing temperatures rangingfrom 50° to 100° C. are preferred. In a particularly preferredembodiment the operation takes place under reflux of the dispersionmedium.

The dispersion time depends on the type of the materials employed butgenerally ranges from several minutes to several hours, e.g., 1 to 24hours.

In order to enhance the deagglomeration, the dispersion (suspension) mayoptionally be treated with ultrasonic waves, intensive mixers orgrinding processes conventional in the field of ceramic materials, e.g.,agitator-ball mills.

Following the completion of the surface-modification, the dispersion(suspension) obtained may either be further processed as such (i.e., forthe production of green bodies or for the coating of substrates) orprior to said further processing the dispersion medium is completely orpartially removed (e.g., up to a desired solids concentration). Aparticularly preferred process for the removal of the dispersion mediumis freeze-drying or freeze-spraydrying.

Following the drying operation, the surface-modified TiN powder mayoptionally be dispersed in a different dispersion medium comprisingwater and/or an organic solvent. For a complete redispersion it hasproved efficient to first modify the TiN with guanidinopropionic acidand to then redisperse it in alcohol or also pure water.

The TiN suspensions obtained according to the process of the presentinvention and the dry surface-modified nanocrystalline TiN powder,respectively have a particle size distribution below 100 nm. They may befurther processed in various ways for the preparation of green bodiesand sintered bodies or coatings, respectively. Extrusion compositionswhich after the extrusion may be sintered to finished molded bodies may,for example, be prepared. For said purpose, there are usually employedfrom 20 to 80, particularly from 30 to 70, and particularly preferredfrom 40 to 60 parts by wt. of surface-modified TiN powder (either assuch or in the form of a dispersion, e.g., as prepared above), from 10to 70, particularly from 20 to 60, and particularly preferred from 30 to50 parts by wt. of dispersion medium, and from 0.5 to 20, particularlyfrom 2 to 15, particularly preferred from 5 to 10 parts by wt. ofadditives selected from binders, plastifiers and mixtures thereof, per100 parts by wt. of extrusion composition.

The mentioned binders and plastifiers are preferably selected frommodified celluloses (e.g., methyl cellulose, ethyl cellulose, propylcellulose, and carboxy-modified cellulose), polyalkylene glycols(particularly polyethylene glycol and polypropylene glycol, preferablyhaving an average molecular of 400 to 50000), dialkylphthalates (e.g.,dimethylphthalate, diethylphthalate, dipropylphthalate, anddibutylphthalate), and mixtures of said substances. Other binders andplastifiers such as, e.g., polyvinyl alcohol, etc., may, of course, alsobe employed.

The above binders and plastifiers are required in order to ensure theformation of an extrudable composition and a sufficient dimensionalstability after the molding operation.

After the thorough mixing of the above components (e.g., in aconventional mixer), a part of the dispersion medium may be removedagain (preferably under reduced pressure) until the extrusioncomposition has the desired solids content. Preferred solids contents ofthe extrusion composition are at least 30%, and particularly at least40% by volume.

Other preferred molding processes are electrophoresis, slip casting,slip casting under pressure, and filter pressing as well as combinationsof electrophoresis and slip casting, slip casting under pressure orfilter pressing; further injection molding, fiber spinning, gel casting,and centrifugation. Compact molded bodies having high green densitiesare obtained in said molding processes. It is also possible to use thesuspensions for coating purposes. Suitable coating processes are dipcoating, spin coating, doctor blade coating, brushing, andelectrophoresis. Suitable substrates are, e.g., metals, ceramicmaterials, hard metals, glass, and cermets.

The green bodies and coatings, respectively produced may then be driedand subjected to a sintering treatment. Surprisingly it has been foundthat the desired densification takes place already at relatively lowtemperatures. Furthermore, surprisingly no sintering additives arerequired. The sintering temperature is usually in the range of from 900°to 1500° C., preferably from 1100° to 1300° C. This is significantlylower than according to the prior art, where temperatures up to 2000°C., sintering additives and possibly also pressure are usually required.

The TiN sintered bodies and coatings, respectively are characterized bya nanoscale structure having grain sizes below 100 nm, a density >95% ofthe theory and a hardness of HV₀.5 >18 GPa.

The TiN produced according to the present invention may, e.g., beemployed as

Ceramic bulk material, e.g., for grinding powders.

Coating material for metals, ceramics and glass for decoration purposes,wear protection, tribological applications, corrosion protection,particularly as coating on cutting tools and grinding agents andgrinding powders, respectively.

Component in ceramics/ceramic composites. As matrix phase, Al₂ O₃, TiC,SiC and Si₃ N₄ may, for example, be envisaged.

Component of nanocomposites.

Sintering aids for coarser TiN and other ceramic materials.

Metal/ceramics composites of the hard material type.

Cermets.

Microporous layers for filtration purposes, e.g.,micro-ultra-nano-filtration and reverse osmosis.

The following examples are to further illustrate the present inventionwithout, however, being a limitation thereof.

EXAMPLE 1

Surface-Modification of TiN in the Nanoscale Range

In 200 ml of a water/ethanol mixture (volume ratio 1:1), 1 g ofguanidinopropionic acid is dissolved. Under continuous stirring, 10 g ofTiN powder are added to said solution. Subsequently, the mixture isheated under reflux at 100° C. for 5 hr. After the reaction time haslapsed the suspension is separated and the filter residue is washed withethanol. The moist powder obtained is dried at 70° C. for 8 hr.

EXAMPLE 2

Redispersion and Formation of a Slurry of the TiN Powder

The surface-modified TiN powder of Example 1 (60 g) is added to 100 mlof water under continuous stirring and intermediate ultrasonictreatment, maintaining the pH of the suspension at a value of about 9 byadding tetrabutyl ammonium hydroxide. This results in a stable slurryhaving a solids content of 37.5% by wt. The particle size ranges from 20to 50 nm.

EXAMPLE 3

The process of Example 2 is repeated with the exception of usingmethanol as redispersion medium instead of water.

EXAMPLE 4

The process of Example 2 is repeated with the exception of using ethanolas redispersion medium instead of water.

EXAMPLE 5

Preparation of Green Body from the TiN Slurry (Slip Casting)

The 37.5% by wt. TiN slurry of Example 2 (50 ml) is poured into a roundPMMA mold (diameter: 40 mm, height: 50 mm, pore size 1 μm). After aholding time of 6 hr a green body having the following dimensions isformed: diameter 40 mm, height 3 mm, green density 40-50% of the theory.

EXAMPLE 6

A green body is produced according to Example 5 with the exception thatadditionally pressure (e.g., 5 bar) is applied in order to reduce thecasting time.

EXAMPLE 7

Sintering of the Green Body

Green bodies produced according to Example 5 are dried under controlledhumidity and temperature in a climatic cabinet. Following the dryingoperation, said bodies are sintered in an argon atmosphere attemperatures between 1100° and 1300° C. The heating rate is 3 K/min upto T=600° C., 20 K/min between 600° C. and the isothermic holdingtemperature. By this sintering treatment the samples reach relativedensities above 95% of the theory and have average grain sizes above 100nm.

EXAMPLE 8

Coating of Al₂ O₃ Substrates

Following the process of Example 1, a 20% by wt. aqueous suspension ofsurface-modified TiN powder is prepared. A dense-sintered Al₂ O₃ plateis coated by dipping it into the suspension. The coated plate is driedand sintered at 1300° C. in an argon atmosphere. Thereby a compact TiNovercoat layer having a thickness of about 5 μm is obtained.

What is claimed is:
 1. A process for producing TiN sintered bodies orcoatings, comprising the steps of:adding TiN powder comprisingagglomerated nanocrystalline TiN particles to a dispersion medium in thepresence of at least one organic surface-modifying agent;deagglomerating said powder by allowing said organic surface-modifyingagent to react or interact with a surface of said nanocrystalline TiNparticles, thereby forming a nanodisperse suspension of surface-modifiednanocrystalline TiN particles; processing said surface-modifiednanocrystalline TiN particles into a green body or coating; andsintering said green body or coating; wherein said dispersion medium isremoved.
 2. Process according to claim 1, wherein said green body orcoating is sintered at a temperature of up to 1300° C.
 3. Processaccording to claim 1, wherein said organic surface-modifying agent has amolecular weight of not more than
 1000. 4. Process according to claim 1,wherein said organic surface-modifying agent is selected from the groupconsisting of aliphatic, saturated or unsaturated C₁ -C₁₂ monocarboxylicacids, polycarboxylic acids, amines of the formula R_(3-n) NH_(n),wherein n=0, 1 or 2 and the radicals R independently represent C₁ -C₁₂alkyl groups, C₄ -C₁₂ β-dicarbonyl compounds, organotitanates,organozirconates, modified alcoholates, and organoalkoxysilanes.
 5. Theprocess according to claim 4, wherein said radicals R independently areC₁ -C₆ alkyl groups.
 6. The process according to claim 4, wherein saidβ-dicarbonyl compounds are C₅ -C₈ β-dicarbonyl compounds.
 7. Processaccording to claim 1, wherein said dispersion medium comprises a mediumselected from the group consisting of water, a polar organic solvent,and a mixture of water and a polar organic solvent.
 8. Process accordingto claim 1, wherein said dispersion medium is present in an amount from20 to 90% by wt. based on the total weight of said dispersion medium,said TiN powder and said organic surface-modifying agent.
 9. The processaccording to claim 8, wherein said dispersion medium is present in anamount of 30 to 80% by weight, based on the total weight of saiddispersion medium, said TiN powder, and said organic surface-modifyingagent.
 10. Process according to claim 1, wherein the weight ratio ofsaid TiN powder to said organic surface-modifying agent ranges from1000:1 to 4:1.
 11. The process according to claim 10, wherein saidweight ratio ranges from 500:1 to 8:1.
 12. Process according to claim 1,wherein said deagglomerating step is carried out at a temperature offrom 20° C. to the boiling temperature of said dispersion medium. 13.The process according to claim 12, wherein said deagglomerating step iscarried out under reflux of said dispersion medium.
 14. Processaccording to claim 1, wherein said dispersion medium is removed byfreeze-drying.
 15. Process according to claim 1, wherein after saiddispersion medium is removed, said surface-modified nanocrystalline TiNparticles are redispersed in a second dispersion medium having acomposition that is the same as or different from the first saiddispersion medium.
 16. The process according to claim 1, wherein saidorganic surface-modifying agent is guanidine carbonate orguanidinopropionic acid.
 17. The process according to claim 1, whereinduring said allowing step wherein said organic surface-modifying agentreacts or interacts with a surface of said nanocrystalline TiNparticles, the reaction or interaction is selected from the groupconsisting of a Bronsted or Lewis acid/base reaction, a Bronsted orLewis complex formation, a Bronsted or Lewis adduct formation, adipole-dipole interaction, a condensation reaction, and a mixturethereof.